I'm a little surprised that this opinion is unanimous.
There are three different possible interpretations of the part of the federal supplemental jurisdiction statute, 28 USC 1367(d), that says that state limitations periods are "tolled" during the pendency of a federal action (and for thirty days thereafter) if the federal court ultimately exercises its discretion to dismiss supplemental state law claims. Justice Werdegar cogently lays out the competing views: the "suspension" position, the "grace period" interpretation, and the "substitution" approach. This issue has confounded state courts pretty much everywhere: around half have adopted the "suspension" approach and the other half the "grace period" position.
Justice Werdegar notes that the text of the federal statute seems to most naturally support the "suspension" approach.
So the California Supreme Court unanimously adopts the competing "grace period" interpretation.
With the various courts roughly evenly split, you'd facially expect seven state court justices to be split fairly evenly themselves. Yet here you have it: A unanimous opinion.
Maybe everyone simply sees it the same way. Or maybe this is a pretty good example of the justices preferring to speak with a unified voice. Especially when the matter is relatively unimportant (at least in the scheme of things) and it's more critical to have a solid, articulated rule than it is for that rule to be precisely right.
Right or wrong, until the United States Supreme Court speaks, or until Congress amends the statute, the rule in California is now clear. If you've got supplemental claims that were dismissed, you've got thirty days after dismissal to refile 'em in state court (or the original limitations period, if longer). Even if you filed your original lawsuit well in advance of the expiration of the limitations period. You don't get that extra time back after dismissal.
That's the rule. Not in lots of states. But -- as of today -- definitely here.